I would argue that one of the great challenges facing the Church today is holiness.
We are, on the whole, not a holy people (unintentional pun aside). We are commanded to be - "...you shall be holy, for I am Holy" says the Lord in Leviticus 11:44 , echoed in First Peter 1:15 "...but as He who called you is holy, be holy in all your conduct as it is written "Be holy, for I am holy." And yet I find that in the larger circle of professing Christians there are very few that one can look at and say "That one is holy." Conceivably this is a problem, if we are supposed to be holy in all of our conduct.
Our traditional view of holiness seems to be rooted in the concept of not doing some things and doing other things - for example, not dancing or drinking or gambling or listening to certain music and instead doing other things such as praying or alms giving or serving. That is not to say that not dancing or drinking or gambling or listening to certain music is not a bad thing any more than doing things like praying or alms giving or serving is a good thing- but in both cases, we define holiness by the behavior we practice rather than something we are, which clearly seems implied by the text.
"Qados" (Kadosh) is the Hebrew word for "holy", describing something which was devoted or dedicated to a particular purpose - the Sabbath day is declared "qados", the people of Israel are declared "qados", the High priest is declared "qados". The equivalent word in the Greek is "hagios" (as in Hagia Sophia, the Church of Holy Wisdom).
God defines himself as set apart - in His case, in nature and in being. We cannot be "holy" in the greater sense of the word, but we can be so in the lesser sense of the word.
With this definition, I would propose that holiness as we have come to understand it takes on an entirely different meaning Our actions do not by themselves make us holy; it is the fact of our lives and how we conduct them that does. We traditionally understand this term in the context of the orders (priests/monks/nuns) or the pastorate where individuals devote their lives to God. The reality of the Scriptures is that all Christians are to be set apart or devoted to God and His service.
But practically speaking, what does holiness look like? There is the rub of course; perhaps not unsurprising as we seem to have so few role models to look to. I might suggest a few guide posts:
1) Avoiding sin: Think of the Old Testament sacrificial system. What was one of the characteristics that the animal sacrifices shared? They were to be free of blemish (check out Leviticus). Blemish implied less than perfection - which is certainly a definition of sin. We are to free of sin in our lives (always work in progress of course) - not that this earns us merit but that it reflects our status as being set apart or devoted to God.
2) A life focused on God: When a thing was devoted to the Tabernacle and then later to the Temple, it existed at that point to serve God: if a pot, then a pot for ashes from the altar or boiling of the flesh of the sacrifice; if a beast of burden, then serving the priesthood. Once something was devoted it almost was never returned to regular service. Likewise once we make the determination to be holy our lives become set apart to serve God always.
What does this look like? It is different for different people and I can no better define it for you than you can for I. Paul's analogy of the body is correct: we are all different parts and fit in differently; what is important is that we recognize that we fit into and serve the Body.
A very simple - extraordinarily simple - item would be choosing something else over God, His clear commands, or His leading. When my life becomes driven by what others think or a hobby that encompasses all my time or even service that is not dedicated to God, I am choosing something else over Him.
3) Doing God's Work and Will - A pastor of days gone by defined holiness as "Thinking what God thinks and willing what God wills." There may be many things that seem murky or hidden to us about God - but there are plenty of places where He is crystal clear on actions or understandings. And those actions or understandings, applied in our lives, will become principles which may inform some of the less clear areas for us. In this sense God is not some kind of cruel author that expects us to follow something that He has hidden away; He has revealed much but we treat them as suggestions rather than as the commands that someone that is devoted or set apart would do.
I wish I could tell you I was better at this than I was - but I struggle too often with lists of do's and do not's as if somehow that would advance my cause, or end up in a sort of bland transcendent stoicism that is not bothered by the life around it. But holiness is not that either: it is passionately alive and involved with every square inch of life around it, even in the pain and rejection and frustration because it belongs to God.
But let us not have any illusion that this holiness, were we to practice it, would inherently lead to any sort of better of the Church or even our own well being. Like the items devoted to Temple service, their whole existence was to point to and serve their Creator. Any growth we see or improvement we realize is purely the grace of God, not a payment on a debt we have incurred in our imagination To the Devoted, their life is only ever about the One they serve, not themselves.